How do I assign a shortcut to something if it isn't listed in User Preferences' shortcut list?
The Input panel in the User Preferences is for searching for shortcuts that already exist. As there isn't a shortcut for Select Pattern already, it won't appear in there.

To add a shortcut, go to the menu where Select Pattern is (Object Mode> Select> Select Pattern...), right-click and choose Add Shortcut. After entering a shortcut it will now be present in the User Preferences when you search for it.